The use of alkaline earth metal salts of organic carboxylic acids as additives for lubricating oil compositions is known. The said salts have a dispersant property so that they, when applied in such composition, ensure that the inside of engine cylinders remains clean and that deposition of carbonaceous products on pistons and in piston grooves is counteracted, so that piston-ring sticking is prevented.
It is also known to prepare basic (or overbased) alkaline earth metal salts of such acids. The overbasing provides an alkaline reserve which, when applied in lubricating oil compositions, reacts with and neutralizes acidic compounds formed during the operation of the engine in which the composition is applied. Hence, sludge which may arise, is maintained dispersed due to the dispersant property of the salt while acids which would enhance sludge formation are neutralized.
In British Patent Specification No. 786,167, a process for the preparation of basic salts is described in which an organic acid is reacted with an excess of an alkaline earth metal oxide or hydroxide in an oil and subsequently carbon dioxide is passed through the reaction mixture to yield basic salts. As suitable acids are mentioned substituted or unsubstituted aliphatic, cycloaliphatic and aromatic acids, comprising carboxylic acids, sulphur-containing acids, phosphoric acids, thio-acids, phenols and partial esters of sulphur- and phosphorus-containing acids.
The incorporation of basic salts into lubricating oil compositions in some cases gives rise to problems. One of these problems is that the lubricating oil compositions tend to form haze on standing. Haze may occur by a phase separation between the basic salt complexes and the base oil, which may lead to precipitation of the complexes. This problem is more pronounced now that many lubricating base oils have been subjected to hydrocracking and/or hydrotreating.
As a solution for this problem it has been proposed in GB-A-1,290,251 to add to the lubricating oil composition an aliphatic carboxylic acid or anhydride having at least 23 aliphatic carbon atoms per carboxy group. Examples of such an acid or anhydride include hydrocarbon-substituted succinic acids or anhydrides having at least 50 aliphatic carbon atoms. The latter acids and anhydrides are described in e.g. US-A-3,219,666.
It has now been found that addition of such acids or anhydrides to a basic salt-containing lubricating oil composition does not always eliminate the haze entirely. Applicants have found that when the basic salt is prepared in the presence of the aliphatic acid or anhydride, surprisingly a stable, clear solution is obtained in which no haze occurs.